1. Field of the Invention
This invention resides in materials and methods for crop protection, and is particularly concerned with soil treatments to control the growth of nematodes and weeds.
2. Description of the Prior Art
The agricultural industry relies on effective control of insects, plant pathogens, nematodes, and weeds, for bountiful and healthy crops. The control of certain forms of nematodes, for example, is an important factor in ensuring crop plant viability in the initial stages of crop growth and likewise for plant productivity and life span in both annual and perennial crops. Effective nematode control is particularly important in view of the ability of nematodes to persist in soil after crop removal.
Known methods for controlling nematodes include crop rotation, fallowing, the use of nematode-resistant crops, and soil fumigation. Of these, soil fumigation is the method that is the most economically feasible and the most widely used, and methyl bromide is the most widely used soil fumigant. Methyl bromide is a highly effective broad spectrum pesticide that is used both in both pre-harvest and post-harvest fumigation. A difficulty with methyl bromide however is its volatility, which results in the release of a significant amount of the chemical to the atmosphere when applied to soil. This reduces the amount of methyl bromide available for pesticidal action in the soil and also causes depletion of the ozone layer due to the reaction of ozone with the bromine atoms that are released when the methyl bromide undergoes photo-oxidation. The high volatility of methyl bromide also limits the effectiveness of this fumigant in heavy soils.
Cyanamide, a term that is used to designate both hydrogen cyanamide and calcium cyanamide, is used in Europe as an herbicide and in various parts of the world as a plant growth regulator. The convenience of using cyanamide as a liquid formulation has increasingly made liquid cyanamide the favored material, and research has shown that hydrogen cyanamide is the active ingredient of both hydrogen cyanamide and calcium cyanamide preparations. From its introduction in the 1950's, hydrogen cyanamide was used as a pre-emergence herbicide and a defoliant. The effectiveness of hydrogen cyanamide in plant protection was attributable at least in part to the hydrolysis of calcium cyanamide to hydrogen cyanamide in moist soil, although the fact that such hydrolysis was occurring was not appreciated at the time. In the environment, hydrogen cyanamide decomposes to urea, followed by the decomposition of urea to ammoniacal forms of nitrogen and carbon dioxide. These metabolites become plant nutrients and their occurrence is mediated by biological organisms and/or physical processes.
The rate at which hydrogen cyanamide formulations decompose can be controlled by the incorporation of a stabilizing agent, and phosphoric acid has commonly been used for this purpose. When drip irrigation was introduced in the 1960's, it was discovered however that the inclusion of phosphoric acid in a hydrogen cyanamide formulation containing water with a high calcium content caused plugging of the drip emitters. Similar plugging was observed in spray systems with small diameter spray nozzles.